This application is the US national phase of International Application No. PCT/GB99/03055, filed 14 Sep. 1999, which designated the U.S., the entire content of which is hereby incorporated by reference.
1. Field of the Invention
This invention relates to the fabrication of optical waveguides.
2. Discussion of Prior Art
One known technique for fabricating optical waveguides is the so-called direct bonding (or direct interfacial bonding) technique.
Direct bonding (DB) is a fabrication technique that uses the Van der Waals forces present when two atomically flat bodies approach each other to create a bond between two bodies. If the bodies are laminas of optical material having appropriate refractive indices, the material laminas can be joined to form waveguiding boundaries.
In one established way to form such a bond the surfaces of two pieces of optical material are polished so as to be very flat (i.e. substantially flat at atomic dimensions). The crystalline structures of the two polished faces are preferably aligned with each other and the polished faces are pressed together. A heat treatment can be useful to encourage a pyroelectric effect and the exchange of electrons between the two surfaces. This gives rise to an electrostatic attraction between the two surfaces, which tends to expel any remaining air or liquid from between the two surfaces. A final annealing step can improve the bond strength further.
A DB bond can be formed irrespective of the lattice constants and orientation of the bodies involved and causes no degradation on the crystalline microstructure or either material. By contacting surfaces in such a non-destructive way, DB preserves the bulk characteristics of each bonded material whilst avoiding possible problems caused by lattice defects, such as increased propagation loss and optical damage.
EP-0598395 describes forming an optical waveguide device by direct bonding of a support substrate and a low refractive index layer on a glass substrate, then etching the glass substrate.
This invention provides an optical waveguide comprising at least a guiding lamina of optical material bonded by direct interfacial bonding to a superstructure lamina of optical material, in which regions of the guiding lamina have modified optical properties so as to define a light guiding path along the guiding lamina characterised in that the waveguide further comprises a second superstructure lamina bonded by direct interfacial bonding to the guiding lamina.
The invention recognises and addresses the shortcomings of previous proposals for the use of DB structures in optical waveguides. In such previous proposals, a flat lamina of a material having a raised refractive index (forming a waveguide xe2x80x9ccorexe2x80x9d) is bonded between two laminas of material having a lower refractive index (forming a waveguide xe2x80x9csuperstructurexe2x80x9d). While this provided a bulk guiding structure, the large lateral dimension of the flat xe2x80x9ccorexe2x80x9d lamina meant that the arrangement was not useful for many waveguiding applications or as a single-mode waveguide.
In contrast, in the invention, regions of the core lamina have modified optical properties so as to define a light guiding path along the core lamina. This can give a greatly increased flexibility of use and allow the guiding path to be much more tightly defined than in previous arrangements.
Although the method is suitable for use with many types of materials, such as glasses, it is preferred that the core lamina is a ferroelectric material, allowing the modified regions to be generated by electrical poling.
A particularly useful ferroelectric material having well-studied optical and electrical properties, is periodically poled lithium niobate (PPLN). PPLN combines a large non-linear coefficient, a widely-controllable phase-matching wavelength, and zero walk-off characteristics that make it an ideal material to achieve quasi-phase matching (QPM) for non-linear frequency conversion. With recent improvements in the efficiency of second-harmonic generation (SHG) within PPLN substrates, it is recognised in the present invention that the use of such a material in an appropriate waveguide geometry formed using the invention can provide a realisation of various compact non-linear devices based on harmonic or parametric generation.
The present method is particularly appropriate for use with PPLN, and has several advantages over other techniques for fabricating waveguides using PPLN such as the so-called xe2x80x9cannealed proton exchangexe2x80x9d technique and the xe2x80x9ctitanium indiffusionxe2x80x9d technique, both of which act on a single PPLN crystal and modify the crystal near the surface in order to create regions of higher refractive index for optical confinement.
Previous experiments investigating the bonding characteristics of PPLN have been directed towards fabricating thick multi-laminated stacks of the material for a large physical aperture, and thus high power applications. In contrast, creating a sufficiently thin lamina of PPLN increases the average pump intensity applied to the domain-inverted structure via optical confinement, and thus allows efficient SHG even at low pump powers. Fabrication of such a device is obtainable by bonding PPLN onto a suitable substrate before precision polishing down to waveguide dimensions, a method which has already been demonstrated in the production of LiNbO3 planar waveguides for electro-optic applications. One of the primary attractions offered by this technique is that the non-linearity and domain characteristics of the PPLN structure after bonding should remain unchanged from the bulk materialxe2x80x94a combination that annealed proton exchange and Ti indiffusion methods are close to achieving, but not yet at their full theoretical efficiencies. A further advantage of the present method is the extra flexibility available when designing devices, as combinations of multiple laminas with different material properties are now possible.
Viewed from a second aspect this invention provides a method of fabricating an optical waveguide, the method comprising the steps of:
(a) bonding, by direct interfacial bonding, a guiding lamina of optical material to a superstructure lamina of optical material;
(b) before, during or after step (a), modifying optical properties of regions of the guiding lamina so as to define a light guiding path along the guiding lamina; characterised in that the method further comprises the steps of:
(c) after steps (a) and (b), removing material from the guiding lamina to reduce the thickness of the guiding lamina (10); and
(d) after step (c), bonding, by direct interfacial bonding, a further superstructure lamina (20) to the guiding lamina.